A water absorbent resin is a white resin represented by a cross-linked polyacrylic acid polymer, and it is general that a form of the water absorbent resin is powder. The water absorbent resin, particularly water absorbent resin powder is used as a sanitary material absorbing agent for a sanitary napkin, a disposable diaper, and the like. Further, the water absorbent resin is used for wide variety of purposes, e.g., a water retaining agent, a dehydrating agent, and the like in an agriculture/horticulture field, a civil engineering and construction field, and a similar field. However, if the water absorbent resin is left for a long time, this raises such a problem that an external factor such as heat and moisture causes the water absorbent resin to be colored yellow or brown. Particularly in the sanitary material field, when the water absorbent resin (powder) of a water absorbing article such as a disposable diaper, a sanitary napkin, and the like is colored, the colored water absorbent resin in a white pulp causes a consumer to regard the resin as having foreign substances, which significantly drops its commercial value as the water absorbing article. Thus, the water absorbent resin is required to be white not only at the time of shipment but also on a permanent basis.
Therefore, there have been proposed several methods each of which prevents the water absorbent resin from being colored even in case where the water absorbent resin is reserved for an extended period of time at high temperature and high humidity. Examples of the methods include: a method in which an inorganic reducer and metal salt are added to the water absorbent resin (Patent Document 1); a method in which sulfinic acid is added as a reducer at the time of polymerization (Patent Document 2); a method in which an aminocarboxylate metal chelating agent and an oxidizing agent or a reducer are added (Patent Document 3); a method in which hydroquinones, i.e., impurities of acrylic acid used in polymerization are controlled (Patent Document 4); a method in which methoxyphenols, i.e., inhibitor of acrylic acid used in polymerization are controlled (Patent Document 5); and the like.
Further, it is natural that the commercial value of the water absorbent resin is decreased by the aforementioned coloring. Also, it is known that: if a metallic foreign substance is slightly included in the water absorbent resin, this causes deterioration of water absorbing performance, so that the commercial value of the water absorbent resin decreases. In order to remove the metallic foreign substance slightly included in the water absorbent resin, a method in which a magnetic line is emitted to the water absorbent resin (specifically, a method using an iron remover) is proposed as a particularly effective method (Patent Document 6). Further, there is proposed a property stabilization method in which a product whose property is below or above a certain property is sorted and the sorted product is mixed with an original product so as to stabilize a property such as an absorbency under load (Patent Document 7).
Further, also a method in which a rotary sorting apparatus is used to remove agglomerates each of which has a large particle diameter (Patent Document 8) is proposed.
[Patent Document 1]
U.S. Pat. No. 6,359,049
[Patent Document 2]
WO2004/084962 (corresponding U.S. Patent Publication No. 2006-089611)
[Patent Document 3]
European Patent No. 1466928 (corresponding U.S. Patent Publication No. 2005-085604)
[Patent Document 4]
U.S. Pat. No. 6,444,744
[Patent Document 5]
U.S. Patent Publication No. 2004-0110914
[Patent Document 6]
U.S. Pat. No. 6,716,894
[Patent Document 7]
U.S. Patent Publication No. 2004-0110006
[Patent Document 8]
U.S. Patent Publication No. 2007-041796